1. Field of the Invention
The present invention relates to an axle driving apparatus comprising a housing, a hydrostatic stepless transmission and an oil filtrating means for filtrating oil fed into the transmission, wherein a hydraulic pump, a hydraulic motor and a center section forming a fluid circuit fluidly connecting the pump and motor constituting the transmission are disposed together in the housing and the oil filtrating means is disposed out of the housing
2. Related Art
Japanese Laid Open Gazettes Nos. Hei 6-84,061, Hei 6-61,555 and Hei 7-63,157 disclose well-known conventional arrangements of oil filtrating means provided on or in such an axle driving apparatus that axles, a hydraulic pump driven by power from an engine and a hydraulic motor driven by oil discharged from the pump for driving the axles are contained in a housing; the pump and motor being fluidly connected with each other through a pair of oil passages within a center section so as to constitute a hydrostatic stepless transmission.
There are important points for providing a hydrostatic stepless transmissional axle driving apparatus with an oil filtrating means for filtrating operating oil for the transmission thereof in that the oil filtrating means should be disposed so as to be easily detachable, preferably without soiling a user's hand because it requires frequent maintenance or exchange and that means for communicating the pair of oil passages within the center section fluidly connecting the hydaulic pump and motor should be provided.
The most suitable manner for enabling the maintenance and exchange of a filter element to be easily done without soiling a hand is the disposal of an oil filter containing a filter element on the exterior of the housing, as disclosed in Hei 6-84,061. The oil filter is provided therein with an oil suction port, an oil discharge port and a filter element interposed between both the ports.
However, referring to the axle driving apparatus disclosed in this cited art, the center section is not soaked in the oil sump in the housing but fixed on one opening end of the housing, so that the hydraulic pump and motor are attached to the inner surface of the center section facing the interior of the housing. The both ports of the oil filter is connected with oil passages bored in the wall of the housing respectively, so that the oil suction port is in communication with an oil sump in a housing serving as an oil source, and the oil discharge port is in communication with the pair of oil passages constituting the fluid circuit of the hydrostatic transmission. In this relation, an oil communicating portion between the oil sump in the housing and the oil suction port of the oil filter and an oil splitting portion between the oil discharge port of the oil filter and the pair of oil passages of the center section must be formed within the wall of the housing or the center section, thereby complicating the processing of the housing or the center section.
Furthermore, such a construction that the center section is disposed out of the housing probably causes the oil leaking between the housing and the center section. The center section of each of the axle driving apparatuses disclosed in Hei 6-61,555 and Hei 7-63,157 is contained in a housing, thereby solving this problem.
However, the center section contained in the housing is soaked in the oil sump therein, so that if the oil filter of the above type is attached on the exterior of the housing, the both ports of the oil filter is spaced from the center section through the oil sump. Then, the oil filtrating means in each of Hei 6-61,555 and Hei 7-63,157 is disposed in the housing in the condition of being soaked in the oil sump, the outlet portions thereof in communication with the oil passages of the center section.
Referring to Hei 6-61,555, the filtrating means is directly attached to the lower surface of the center section. The housing is open at the bottom thereof and an oil pan casing is attached to the opening bottom. The filtrating means is disposed in the oil pan casing. In this construction, the center section forms a communicating passage toward the oil filtrating means in the bottom portion thereof. For maintenance or exchange of the oil filtrating means, the oil pan casing must be detached.
The cylindrical shaped oil filtrating means disclosed in Hei 7-63,157, is substantially horizontally extended between the side portion of the housing and the center section through the oil sump in the housing. The outer end of the oil filtrating means is inserted in an opening of the housing and a lid is pushed into the opening from outside of the housing against the outer end of the oil filtrating means, so that, when the lid is pulled out, the means can be pulled out from the interior of the housing.
These oil filtrating means are soaked in the oil sump in the housings, thereby compelling a user to have his/her hand soiled while detaching the means. The former cited axle driving apparatus requires the complicated removal of bolts from the oil pan casing for detaching the means. Also, a vehicle provided with this apparatus hits at the downward extending oil pan portion thereof against an obstacle on the ground with rather high possibility. It is hopeful for an axle driving apparatus to be constructed without such a downward extending portion, so as to provide a vehicle with a large ground clearance as much as possible. The latter cited axle driving apparatus which is provided with the oil filtrating means substantially horizontally disposed in the housing, can solve the problem, however, holds such another problem that, when a new replacing filtrating means is inserted into the housing, it is difficult to clear the position of the inner utmost end of the means with respect to the engaging portion of the center section.
As a result, an axle driving apparatus provided with a hydrostatic stepless transmission, whose center section is contained in a housing for prevention of oil leaking, prefers an oil filtrating means like the above mentioned oil filter disposed out of the housing so as to enable easy attachment and detachment thereof without soiling a user's hand. Furthermore, if the means is disposed on a lateral side of the housing, the apparatus can be vertically compact, thereby enabling a vehicle to have a large ground clearance. However, such an apparatus requires an oil splitting system where the oil discharged from the outlet port of the filtrating means disposed out of the housing is separated into the two flows toward the respective pair of oil passages of the center section disposed in the housing. If the oil splitting system is formed by the housing or the center section, they are complicated in its processing, thereby also becoming expensive. Furthermore, when a trouble is generated in the oil split system, the housing or the center section itself must be repaired or exchanged in the worst, thereby worsening maintenance. Hence, a detachable member having the oil split system, which is independent to the housing and the center section, is preferred. Conventionally, such a member has not been known.